Photoconductive alloy of arsenic,antimony and selenium



Aug. 18,1970 J CERLON ET AL 3,524,745

PHOTOCONDUCTIVE ALLOY OF ARSENIC, ANTIMONY AND SELENIUM Filed Jan. 13, 1967 PLATE 4 40M A -Sb-SQ +1 PLATE 3 4OMSb s8 PLATE l 40H Se FIG. 2

WAVELENGTH INVENTORS SPECTRAL RESPONSE CURVES PETER J. CERLON MARK B. MYERS BY EVAN J. FELTY ATTORNEYS United States Patent Olfice York Filed Jan. 13, 1967, Ser. N0. 609,126 Int. Cl. G03g 5/00 US. Cl. 96-1.5 17 Claims ABSTRACT OF THE DISCLOSURE A vitreous photoconductive alloy comprising arsenic, antimony and selenium.

BACKGROUND OF THE INVENTION This invention relates in general to the art of xerography, and in particular, to a new photosensitive element.

In the art of xerography it is usual to form an electrostatic latent image on a member or plate which comprises a conductive backing such as, for example, a metallic surface having a photoconductive insulating layer thereon. A suitable plate for this purpose is a metallic member overcoated with a layer of vitreous selenium. Such a plate is characterized by being capable of receiving a satisfactory electrostatic charge and selectively dissipating such a charge when exposed to a light pattern, and in general, is largely sensitive to light in the blue-green spectral range.

Vitreous selenium for the most part has become the standard in commercial xerography, but many of its properties can be improved by the addition of alloying elements which enhance such properties as spectral response, light sensitivity, thermal stability, etc. U.S. Pats. 2,803,542 to Ullrich and 2,822,300 to Mayer et al. both show the advantages of modifying vitreous selenium by the addition of appreciable amounts of arsenic in order to yield a wider range of spectral sensitivity, increase the overall photographic speed, and in general improve the stability of the photoconductive layer.

Although vitreous selenium shows a satisfactory sensitivity, the need for photoconductors exhibiting increased sensitivity and spectral response exceeding those of vitreous selenium is needed for high speed processes which require a plate having a very high degree of sensitivity or panchromativity due to the short time factor in rapid cycling.

A step in this direction involves the addition of antimony in appreciable amounts to selenium, such as that set forth in copending application Ser. No. 566,593 filed on July 20, 1966. Vitreous alloys of antimony and selenium have been found to yield a photosensitive composition having a sensitivity factor up to 12 times greater than that of vitreous selenium, and in addition, having a relative response up to 3 times that of vitreous selenium in the blue-green spectral range. Although the antimonyselenium system has enhanced xerographic speed, it does suffer from one disadvantage in that there is a relative lack of thermal stability with respect to crystallization.

SUMMARY OF THE INVENTION It is, therefore, an object of this invention to provide an improved system for utilizing a novel ternary photoconductive alloy which overcomes the above noted disadvantages.

It is another object of this invention to provide a novel photoconductive composition of arsenic-selenium-antimony which will combine the thermal stability of the 3,524,745 Patented Aug. 18, 1970 arsenic-selenium system with the speed of the antimonyselenium system.

It is a further object of this invention to provide a system utilizing a photoconductor having improved Xerographic properties.

It is yet another object of this invention to provide an improved photoconductor having high spectral response.

It is a further object of this invention to provide a xerographic plate having a high sensitivity factor.

It is a further object of this invention to provide an improved photosensitive element having improved thermal stability With respect to crystallization.

It is yet another object of this invention to provide a novel composition having enhanced photoconductive properties.

The foregoing objects and others are accomplished in accordance with this invention by providing a novel vitreous arsenic-antimony-selenium alloy for use as a photoconductor. These alloys are prepared in a manner similar to those vitreous photoconductive alloys of the arsenicselenium system such as those described in U.S. Pats. 2,803,542 and 2,822,300, already mentioned above, and in copending application Ser. No. 566,593. It has been discovered that a vitreous alloy of arsenic, antimony and selenium in an effective range of up to about 50 at. percent (48.7 wt. percent) arsenic, about 0.1 to 22 at. percent (0.15 to 31.0 Wt. percent) antimony, and not less than about 40 at. percent (38.9 wt. percent) of selenium, yields a photosensitive composition having a sensitivity factor up to about 12 times that of vitreous selenium, with enhanced thermal stability which can be controlled to be greater than that of vitreous selenium. A preferred range of about up to about 45 at. percent (43.7 Wt. percent) arsenic, up to about 13 at. percent (18.8 wt. percent) antimony, and not less than about 55 at. percent (56.3 wt. percent) selenium has been found to yield the combination of optimum thermal stability and sensitivity. The ratio of arsenic to antimony necessary in order to main tain maximum thermal stability should be in the order of about 2 to 4 at. percent of arsenic or higher for about every 1 at. percent of antimony. In any case, the arsenic should be present in an amount of at least about 0.5 at. percent (0.6 wt. percent) in order to impart appreciable thermal stability, while the antimony should be present in an amount of at least about 0.1 at. percent (0.15 wt. percent) to yield the desired sensitivity of spectral respouse.

The advantages of this improved photosensitive composition will become apparent upon consideration of the following disclosure of this invention; especially when taken in conjunction with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a portion of the ternary diagram for antimony, arsenic and selenium.

FIG. 2 is a series of spectral response curves for a group of photoconductors including a ternary alloy of this invention,

In FIG. 1, the ternary diagram illustrates the areas in which the alloy of antimony-arsenic-selenium exhibits the desired photoconductive properties and thermal stability. The area below Curve ABC-D represents compositions having a preferred thermal stability. The area below Curve EFGH, which also embraces the area below A-BCD, includes additional compositions having sirable xerographic properties than those below Curve A-BCD.

Dotted lines I and K represent the boundaries for the minimum amounts of arsenic and antimony, respectively,

3 which must be present in the ternary alloy. As shown by the ternary diagram, the arsenic should be present in an amount of at least about 0.5 at. percent (0.6 wt. percent) with the amount of antimony being at least about 0.1 at. percent (0.15 wt. percent).

The arsenic-antimony-selenium vitreous alloys of this invention may be prepared by any suitable techniques. Typical techniques are conventional single source evaporation and flash evaporation. In both evaporation techniques it is preferred that a prereaction of the constituents take place before evaporation in that it results in the formation of s'elenide compounds with more similar vapor pressures than the elemental constituents. The starting alloys are prepared by weighing the elemental arsenic, antimony, and selenium, and vacuum sealing them in a silica glass ampul. The materials are heated at 600 C. for several hours then air cooled to room temperature. Depending on the composition, the cooled alloy is completely poly-crystalline, a mixture of crystalline and amorphous phases, or completely amorphous. The prereacted alloy is then ball milled to a fine particulate size of less than about 1 millimeter in diameter.

In single source evaporation of the appropriate amount of the prereacted alloy is placed in a heated shallow fiat crucible or boat which is maintained in a vacuum chamber under any suitable vacuum conditions such as from about to 10* millimeters of mercury.

The crucibles may be made of any inert material such as quartz, molybdenum, or ceramic lined metal. The arsenic-antimony-selenium alloy is maintained at a temperature to insure sufficient vapor for deposition in reasonable time. This temperature usually exceeds the melting point of the alloy. A total evaporation time of about minutes at a temperature of about 400 C. under the above mentioned vacuum conditions results in the formation of an alloy layer about 20 to 40 microns in thickness.

A substrate is supported above the heating crucibles upon which the arsenic-antimony-selenium is evaporated. The substrate is maintained at a relatively low temperature. Suitable substrate temperatures are from about 60 to 150 C.

Another typical method of evaporation includes flash evaporation under vacuum conditions similar to those defined in single source evaporation, wherein the prereacted alloy of arsenic-antimony-selenium having a particle size of about less than 1 millimeter in diameter is selectively dropped into a heated inert crucible maintained at a temperature from about 450 to 660 C. The vapors formed by heating the mixture are evaporated upward onto a substrate supported about the crucible. The alloy is dropped at a rate sufiicient to prevent the formation of an alloy pool in the crucible, and thus minimize the problem of fractional vaporization. The substrate is maintained at a temperature in the range of about 60 to 150 C. This procedure is continued until the desired thickness of the vitreous arsenic-antimonyselenim photoconductor has been formed on the substrate.

The alloys of this invention may be conveniently formed on any substrate, conductive or insulating. This may be a metal plate such as brass, aluminum, gold, platinum, steel or the like. The support member may be of any convenient thickness, rigid or flexible, in the form of a sheet, a web, a cylinder or the like, and may be coated with a thin layer of plastic. It may also comprise such materials as metallized paper, plastic or plastic covered with a thin coating of aluminum or copper iodide, or glass coated with a thin layer of partially transparent copper iodide, tin oxide, or gold. In certain cases, after formation to the alloy, the substrate may even be dispensed with, if desired.

The thickness of the arsenic-antimony-selenium photoconductor alloy layer is not critical. The layer can be as thin as about 0.1 micron, or as great as 300 microns or more, but for most applications the thickness will generally be from about 20 to 80 microns.

In another embodiment, the photoconductive alloy of this invention may be doped with a halogen such as iodine, chlorine or bromine in order to reduced residual voltage and in general enhance its electrical characteristics. The halogen dopant is usually present in amounts from about 10 parts per million up to 2 percent by weight.

In a further embodiment of this invention, the arsenicantimony-selenium alloy may be utilized in a layered configuration. Typical configurations include a relatively thin layer of from about 0.1 to 5 microns of arsenicantimony-selenium alloy over a relatively thicker layer of vitreous selenium. Another typical structure includes an alloy of arsenic-selenium such as that disclosed in U.S. Pat. 2,822,300 which may be used in place of selenium in the two layered configuration defined above. It should be understood that both layers of a two layered structure may be doped with a suitable halogen in order to enhance the electrical characteristics. Structures having more than two photoconductive layers are also included within the scope of this invention. The arsenic-antimonyselenium alloy layer may also be formed at the substrate interface of a transparent substrate, and imaged by exposure through the substrate. In this embodiment the arsenic-antimony-selenium alloy may be used alone or in conjunction with other photoconductive layers such as those defined above.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following examples further specifically define the present invention with respect to a method of making an arsenic-antimony-selenium photosensitive element. The percentages in the disclosure, examples, and claims are by Weight unless otherwise indicated. The examples below are intended to illustrate the various preferred embodiments of making an arsenic-antimony-selenium photoconductor.

Example I A starting alloy is prepared by weighing elemental arsenic, antimony and selenium in the ratio of 18 wt. percent arsenic, 1 wt. percent antimony, and 81 wt. percent selenium and vacuum sealing the mixture in a silica glass ampul. The mixture is then heated to about 600 C. for about 2 hours in a rocking furnace and then cooled to room temperature. The resultant alloy is a solidified polycrystalline matrix of intermixed phases which is then ball milled into a relatively fine material less than about 1 mm. in diameter.

Example II A 40 micron alloy film of vitreous arsenic-antimonyselenium on an aluminum substrate comprising about 1 wt. percent antimony, 18 wt. percent arsenic, and 81 wt. percent selenium is prepared as follows: The prereacted alloy of Example I is placed in a shallow fiat 2 x 4 inch ceramic coated metal boat. The particulate alloy is evenly distributed over the surface of the boat. The boat is then placed into a vacuum chamber. A 4 x 5 inch aluminum plate is thoroughly cleaned and suspended in the vacuum chamber about 12 inches above the boat and maintained at a temperature of about 65 C. with a water cooled platen. The chamber is evacuated into a vacuum of about 10- millimeters of mercury. The arsenic-antimony-selenium alloy is then evaporated onto the aluminum substrate by heating the boat to a temperature of about 400 C. for about 20 minutes. The boat is then cooled to room temperature, the vacuum broken, and the arsenic-antimonyselenium alloy coated aluminum plate removed from the vacuum chamber.

Due to fractionalization during evaporation, the outer surface of the alloy layer of the plate formed in Example II contains a greater percentage of antimony than the interior of the alloy film, with the substrate surface of the alloy layer containing the least amount of antimony. These determinations were made by electron probe analy- SIS.

Example III The plate formed by the method of Example II is then imaged as follows in a xerographic mode: The plate is corona charged to a positive potential of about 300 volts, and then exposed to a 100 watt tungsten light source at a distance of about 16 inches for about /2 second to form a latent electrostatic image on the surface of the plate. The latent image is then developed by cascading electroscopic marking particles across the surface containing said image. The image is transferred to a sheet of paper and heat fused to make it permanent. An excellent reproduction of the original is obtained by this method.

Example IV A 25 micron layer of a vitreous arsenic-antimonyselenium alloy on an aluminum substrate is formed as follows: A prereacted alloy comprising about 12 wt. percent antimony, 28 wt. percent arsenic, and 60 wt. percent selenium is prepared by the method of Example I. The prereacted alloy is placed in a brass hopper containing a copper chute adapted to deliver the particulate alloy into a quartz crucible maintained below said hopper. The quartz crucible is surrounded with a resistance heater adapted to control the crucible at a temperature of about 600 C. An aluminum substrate is placed on a water cooled platen maintained at a temperature of about 70 C. The aluminum substrate and platen are positioned about 12 inches above the quartz crucible. A bell jar is then placed over the hopper, crucible, and substrate, and evacuated to a vacuum of about mm. of mercury, and the quartz crucible heated to a temperature of about 600 C. When a quartz crucible is brought to 600 C., the hopper door below the chute leading to the quartz crucible is opened allowing a small sample of the arsenic-antimonyselenium alloy to pour into the quartz crucible. The alloy mixture rapidly evaporates causing vapors of arsenic, antimony and selenium to come into contact with the suspended aluminum substrate. This process is continued for about 2 hours at which time a 25 micron layer of a vitreous antimony-arsenic-selenium alloy containing about 12 wt. percent antimony, 28 wt. percent arsenic, and 60 wt. percent selenium has been formed on the aluminum substrate. The crucible is then allowed to cool at room temperature, the vacuum broken, and the coated substrate removed from the chamber.

Example V The alloy formed by Example IV is then imaged in the manner set forth in Example III. An excellent copy of the original is obtained by this method.

Examples VI-IX A series of four plates are prepared for a comparison of spectral response. All four plates contain a 40 micron layer of a photoconductor on an aluminum substrate. Plate No. 1 comprises a 40 micron layer of vitreous selenium made according to the method set forth in US. Pat. 2,970,906. Plate No. 2 comprises a 40 micron layer of an 18 wt. percent arsenic, 82 wt. percent selenium vitreous alloy doped with about 1000 ppm. of iodine formed by the method set forth in copending US. patent application 516,529, filed on Dec. 27, 1965. Plate No. 3 comprises a vitreous alloy of 14 wt. percent antimony and 86 wt. percent selenium made according to the method as set forth in copending US. patent application 566,593 filed on July 20, 1966. Plate No. 4 comprises the ternary alloy of Example I comprising about 1 wt. percent antimony, 18 wt. percent arsenic, and 81 wt. percent selenium which is doped with about 1000 p.p.m. of iodine. The spectral response curves for each of plates 1-4 are then plotted and illustrated in FIG. 2.

In FIG. 2, the spectral response of the four plates is plotted at dilferent wavelengths. E represents the energy in ergs/cm. required to discharge plate approximately 25 percent. The plates were each tested at the various wavelengths by charging each plate to an initial field potential of about 15 volts per micron, and exposing each plate at the particular Wavelength to approximately 2X 10 photons per cm. second. As shown from the curves, the arsenic-antimony-selenium plate of this invention at 4000 angstroms exhibits approximately twice the spectral response as that of vitreous selenium and the arsenicselenium alloy, and considerably greater response than that of the antimony-selenium binary alloy.

As can be seen from above, arsenic-antimony-selenium alloys prepared by this invention have an extended spectral response all the way out to about 7000 angstroms and enhanced response in the shorter wavelength region. In addition, these alloys have improved thermal stability not attainable by that of antimony-selenium photoconductors, and in preferred ratios of arsenic to antimony, show a thermal stability exceeding that of vitreous selenium.

Although specific components and proportions have been stated in the above description of the preferred embodiment of this invention, other suitable materials and procedures such as those listed above, may be used with similar results. In addition, other materials may be added which synergize, enhance, or otherwise modify the properties of these plates.

Other modifications and ramifications would appear to those skilled in the art upon reading the disclosure. These are intended to be included within the scope of thi invention.

What is claimed is:

1. A thermally stable vitreous photoconductive alloy of arsenic, antimony, and selenium bounded by the area below the Curve ABCD of FIG. 1, and comprising at least about 0.5 atomic percent arsenic, and 0.1 atomic percent antimony.

2, A vitreous photoconductive alloy of arsenic, antimony, and selenium bounded by the area below Curve EFG-H of FIG. 1, and comprising at least about 0.5 atomic percent arsenic, and 0.1 atomic percent antimony.

3. A photosensitive element having a photoconductive insulating layer, said layer comprising a vitreous alloy of arsenic, antimony and selenium in a concentration of about 0.5 to 50 atomic percent arsenic, 0.1 to 22 atomic percent antimony and the balance substantially selenium in an amount of at least about 40 atomic percent.

4. The element of claim 3 in which the arsenic concentration is about 0.5 to 45 atomic percent and the antimony is present in an amount from about 0.1 to 13 atomic percent.

5. A photosensitive element comprising a supporting substrate, a photoconductive insulating layer overlaying said substrate with said layer comprising a vitreous alloy of arsenic, antimony and selenium in a'concentration of about 0.5 to 50 atomic percent arsenic, 0.1 to 22 atomic percent antimony, and the balance substantially selenium in an amount of at least about 40 atomic percent.

6. The element of claim 5 wherein the substrate is electrically conductive.

7. A photosensitive element comprising a support member, a first photoconductive layer overlaying said support member, and a second photoconductive layer comprising an alloy of arsenic, antimony and selenium overlaying said first photoconductive layer, said second photoconductive layer having an alloy composition comprising about 0.5 to 50 atomic percent arsenic, 0.1 to 22 atomic percent antimony, and the balance substantially selenium in an amount of at least about 40 atomic percent.

8. The element of claim 7 where insaid first photoconductive layer comprises vitreous selenium.

9. The element of claim 7 wherein said first photoconductive layer comprises an arsenic-selenium alloy.

10. The element of claim 7 wherein the first photoconductive layer is doped with a halogen.

11. The element of claim 7 wherein the arsenic-antimony-selenium layer is doped with a halogen.

12. The element of claim 7 wherein a plurality of photoconductive layers are overlayed with an alloy layer of arsenic-antimony-selenium.

13. A method of imaging comprising:

(a) providing a xerographic plate having an electrically conductive support member and a photoconductive insulating layer thereon, said layer comprising a vitreous alloy of arsenic, selenium, and antimony, with arsenic being present in an amount from about 0.5 to 50 atomic percent, antimony in an amount from about 0.1 to 22 atomic percent and the balance substantially selenium in an amount of at least about 40 atomic percent. (b) forming an electrostatic image on said plate; and (c) developing said image to make it visible. 14. A method of imaging comprising: (a) providing a xerographic plate having an electrically conductive support member and a photoconductive insulating layer thereon, said layer comprising a mony-selenium layer is overcoated with a second photoconductive layer.

17. The method of claim 15 wherein the arsenic-antimony-selenium layer is overcoated with a plurality of photoconductive layers.

References Cited UNITED STATES PATENTS vitreous alloy of arsenic, selenium, and antimony,

with arsenic being present in an amount from about 3 96-15 0.5 to 50 atomic percent, antimony in an amount 2962376 1/1960 96-45 from about 0.1 to 22 atomic percent, and the balance 66 962 3 en 96 1'5 selenium in an amount of at least about 40 atomic 1 ar can 96-45 25 age; 2x22; a -h e21;

rang am (bgaisillgigagtlggcyl' uniformly electrostatlcally charging 3,355,289 11/1967 Han et a1 96*1-5 X 3 372 294 3/1968 Fisher 96-15 X (c) exposing said plate to a pattern of activating radiation thereby forming a latent electrostatic image; and 3427157 2/1969 cerlon 96 1'5 X (d) developing said image to make it visible.

GEORGE F. LESMES, Primary Examiner C. E. VAN HORN, Assistant Examiner US. Cl. X.R.

15. A method of imaging comprising:

(a) providing a xerographic plate having a substantially transparent electrically conductive support member, and a photoconductive insulating layer thereon, said layer comprising a vitreous alloy of arsenic, antimony and selenium, with arsenic being present in an 

